Vsb transmission system

ABSTRACT

A vestigial sideband (VSB) modulation transmission system and a method for encoding an input signal in the system are disclosed. According to the present invention, the VSB transmission system includes a convolutional encoder for encoding an input signal, a trellis-coded modulation (TCM) encoder for encoding the convolutionally encoded signal, and a signal mapper mapping the trellis-coded signal to generate a corresponding output signal. Different types of the convolutional encoders are explored, and the experimental results showing the performances of the VSB systems incorporating each type of encoders reveals that a reliable data transmission can be achieved even at a lower input signal to noise ratio when a convolutional encoder is used as an error-correcting encoder in a VSB system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital communication system, andmore particularly, to a vestigial sideband (VSB) modulation transmissionsystem including a TCM (Trellis-Coded Modulation) encoder and anadditional 1/2 rate convolutional encoder having a superior statetransition property when connected to the TCM encoder in the system.

2. Background of the Related Art

The TCM coded 8-VSB modulation transmission system has been selected asa standard in 1995 for the U.S. digital terrestrial televisionbroadcasting, and the actual broadcasting incorporating the system hasstarted since the second half of the year 1998.

In general, a digital communication system performs error correctingprocesses to correct the errors occurred at the communication channels.The total amount of the transmitting data is increased by such errorcorrecting coding processes since it creates additional redundancy bitsadded to the information bits. Therefore, the required bandwidth isusually increased when using an identical modulation technique.Trellis-coded modulation (TCM) combines multilevel modulation and codingto achieve coding gain without bandwidth expansion. Also an improvedsignal to noise ratio can be achieved by using the trellis-codedmodulation (TCM) technique.

FIG. 1A and FIG. 1B illustrate a typical TCM encoder used in a typicalATSC 8-VSB system and corresponding set partitions used by the TCMencoder. According to the FIG. 1A, an input bit d₀ is output as c₁ andc₀ after trellis-coded modulation, and then a subset is selected among(−7,1), (−5,3), (−3,5), and (−1,7). Thereafter, an input bit d₁ selectsa signal within the selected subset. In other words, when d₁ and d₀ areinputted, one of eight signals (−7,−5,−3,−1,1,3,5,7) is selected by c2,c1, and c0 generated by the TCM encoder. d1 and d0 are called an uncodedbit and a coded bit, respectively.

FIG. 1B illustrates the set partitions used by the TCM encoder used inthe ATSC 8-VSB system. Eight signal levels are divided into foursubsets, each of which including two signal levels. Two signals areassigned to each subset such that the signal levels of each subset areas far as possible from each other as shown in FIG. 1B.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a VSB transmissionsystem and a method for encoding an input signal in the VSB transmissionsystem that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

An object of the present invention is to provide a VSB transmissionsystem that can transmit data reliably even at a lower signal to noiseratio and can have an optimal state transition property when connectedto the TCM encoder by using a 1/2 rate convolutional encoder as anadditional error correcting encoder in the system.

Another object of the present invention is to provide a method forencoding an input signal in a VSB modulation transmission systemenabling a data sender to achieve more reliable data transmission at alower signal to noise ratio and to have an optimal state transitionproperty of a 1/2 convolutional encoder, which is concatenated to theTCM encoder for error correcting in the system.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, avestigial sideband (VSB) modulation transmission system includes aconvolutional encoder encoding an input signal; a trellis-codedmodulation (TCM) encoder encoding the convolutionally encoded inputsignal; and a signal mapper mapping the trellis-coded input signal togenerate a corresponding output signal.

In another aspect of the present invention, a vestigial sideband (VSB)modulation transmission system includes a 1/2 rate convolutional encoderencoding an input signal to generate first and second output signals; a2/3 rate trellis-coded modulation (TCM) encoder encoding the first andsecond output signals to generate third, forth and fifth output signals;and a signal mapper mapping the third, forth, and fifth output signals.

There are three different types of 1/2 rate convolutional encoders thatcan be used in this aspect of the present invention. The first typeincludes a plurality of multipliers, each i th multiplier multiplyingthe input signal by a constant k, to generate an i th multiplier value;a plurality of memories, a first memory storing the previous secondoutput value as a first memory value and each i+1 th memory storing ani+1 th memory value obtained by adding an i th memory value stored in ai th memory and the i th multiplier value; and a plurality of adders,each i th adder adding the i th memory value and the i th multipliervalue, where i=1, 2, 3, . . . , n , and a n+1 th memory value stored ina n+1 th memory is the second output signal.

The second type of the 1/2 rate convolutional encoder includes a firstmemory storing the input signal as a first memory value; a second memorystoring the first memory value as a second memory value; a first adderadding the input signal and the second memory value to generate thefirst output signal; and a second adder adding the input signal and thefirst and second memory values to generate the second output signal.

Finally, the third type of the 1/2 rate convolutional encoder includes afirst memory storing the previous second output value as a first memoryvalue; an adder adding the input signal and the first memory value; anda second memory storing a result from the adder as a second memoryvalue, the second memory value being the second output signal.

In another aspect of the present invention, a method for encoding aninput signal in a vestigial sideband (VSB) modulation transmissionsystem includes the steps of encoding the input signal by theconvolutional encoder; encoding the convolutionally encoded input signalby the TCM encoder; and generating a final output signal my mapping thetrellis-coded input signal.

In a further aspect of the present invention, a method for encoding aninput signal in a vestigial sideband (VSB) modulation transmissionsystem includes the steps of generating first and second output signalsby encoding the input signal using the 1/2 convolutional encoder;generating a third, forth, and fifth output signals by encoding thefirst and second output signals using the 2/3 rate TCM encoder; andgenerating a final output signal by mapping the third, forth, and fifthoutput signals.

The second output signal can be generated using three different methodsin the last aspect of the present invention described above. The firstmethod for generating the second output signal includes the steps ofmultiplying the input signal by a constant k, to generate an i thmultiplier value for i=1, 2, 3 . . . n ; storing the previous secondoutput value as a first memory value; and storing an i+1 th memory valueobtained by adding an i th memory value and the i th multiplier valuefor i=1, 2, 3 . . . n , where the second output signal is an n+1 thmemory value.

The second method for generating the second output signal includes thesteps of storing the input signal as a first memory value; storing thefirst memory value as a second memory value; generating the first outputsignal by adding the input signal and the second memory value; andgenerating the second output signal by adding the input signal and thefirst and second memory values.

Finally, the third method for generating the second output signalincludes the steps of storing the previous second output value as afirst memory value; adding the input signal and the first memory value;storing the value resulted from the adding step as a second memoryvalue; and outputting the second memory value as the second outputsignal.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1A illustrates a typical trellis-coded modulation (TCM) encoderused in a ATSC 8VSB transmission system according to the related art;

FIG. 1B illustrates set partitions used by a typical TCM encoder of aATSC 8VSB transmission system according to the related art;

FIG. 2 illustrates an error correcting encoder concatenated to a 2/3rate TCM encoder in a ATSC 8-VSB transmission system according to thepresent invention;

FIG. 3A illustrates a 1/2 rate convolutional encoder concatenated to a2/3 TCM encoder to be used as an error correcting encoder in a ATSC8-VSB transmission system according to the present invention;

FIG. 3B illustrates 2/3 and 1/3 rate convolutional encoders used as anerror correcting encoder in a ATSC 8-VSB transmission system accordingto the present invention;

FIG. 4 illustrates a first type of a 1/2 rate convolutional encoderconcatenated to a 2/3 TCM encoder in a ATSC 8-VSB transmission systemaccording to the present invention;

FIG. 5A illustrates a second type of a 1/2 rate convolutional encoderused in a ATSC 8-VSB transmission system according to the presentinvention and its corresponding state transition diagram;

FIG. 5B illustrates a third type of 1/2 rate convolutional encoder usedin a ATSC 8-VSB system according to the present invention and itscorresponding state transition diagram;

FIG. 6 illustrates a VSB receiving system corresponding to a ATSC 8-VSBtransmission system according to the present invention;

FIG. 7A illustrates Euclidean distances of a set of output signalsgenerated from the 1/2 rate convolutional encoder shown in FIG. 5A;

FIG. 7B illustrates Euclidean distances of a set of output signalsgenerated from the 1/2 rate convolutional encoder shown in FIG. 5B; and

FIG. 8 illustrates performances of ATSC 8-VSB transmission systems wheneach of the 1/2 rate convolutional encoders shown in FIG. 5A and FIG. 5Bis used,

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 illustrates a VSB transmission system in which an errorcorrecting encoder is concatenated to a 2/3 rate TCM encoder accordingto the present invention. By adding an additional error correctingencoder to the 2/3 rate TCM encoder in the VSB system, it is possible toachieve a reliable data transmission even at a lower signal to noiseratio than that of the conventional ATSC TCM coded 8VSB system. In thepresent invention, a 1/2 rate convolutional encoder is used for theadditional error correcting encoder. In addition, a multiplexer locatedbetween the error correcting encoder and the 2/3 rate TCM encoderclassifies the data received from each of the error correcting encoderand a ATSC encoder and inputs each data to the TCM encoder. Theadditional error-corrected data will be regarded as an error by the ATSCreceiver and will be discarded.

FIG. 3A and 3B illustrate a 1/2 rate encoder used as an additional errorcorrecting encoder shown in FIG. 2. According to FIG. 3A, an input bit uis processed in the 1/2 rate encoder to generate two output bits d₁ andd₀, and these are inputted to a 2/3 rate TCM encoder. In FIG. 3B, eachof 2/3 and 1/3 rate encoders is connected to a 2/3 rate TCM encoder.Since the bit error rate of uncoded bits u₁ is lower than that of acoded bit u₀, the encoder having a higher code rate is used for u₁, andthe other encoder is used for u₀. This will compensate the differencebetween two input bits u₀ and u₁, In addition, the 2/3 and 1/3 rateencoders can be considered as being a 1/2 rate encoder since it hasthree input bits and six output bits. Thus, combining encoders havingdifferent code rates can reduce the bit error rate of the whole system.As a result, the additional encoder can be any one of the 1/2 rateencoder and the combination of the 2/3 rate encoder and the 1/3 rateencoder shown in FIG. 3A and FIG. 3B, respectively. By adding theadditional encoder, the performance of the system can be enhanced, andthis will be shown later in this section. Considering the signal mappingof the TCM encoder, the error correcting encoder must be designed sothat it has the optimal state transition property when connected to theTCM encoder.

FIG. 4 illustrates a first type of a 1/2 rate convolutional encoderconcatenated to a 2/3 TCM encoder in a VSB transmission system accordingto the present invention. The 1/2 rate convolutional encoder receives aninput bit u and generates a first output bit d₁ by bypassing u. A secondoutput bit d₀ is the value of the N+1 th memory m_(i+1). The 1/2 rateconvolutional encoder includes N multipliers, N adders, and N+1memories. The first memory m₁ stores a previous second output value, thefirst multiplier g₁ multiplies the input bit u by a first constant k₁,and the first adder adds the outputs from g₁ and m₁. Similarly, each i+1th memory m_(i+1) stores the output from the i th adder, the i thmultiplier g_(i) multiplies the input bit u by an i th constant k_(i),and the i th adder adds the outputs from g_(i) and m_(i), where i=2, 3,4, . . . ,N . Finally, the N+1 th memory m_(i+1) stores the output fromthe N th adder. Then the value stored in m_(i+1) is output as a secondoutput bit (current). In addition, the second output bit (current) isfeedback to the first memory m₁ for calculating a next second outputvalue. N can be greater than or equal to two and can be determined asone wishes to design the system. As shown in the FIG. 4, the 1/2 rateconvolutional encoder receives u and outputs d₀ and d₁ . d₀ and d₁ thenbecome the output bits c₁ and c₂ of the TCM encoder. Therefore, whend₁d₀=00, c₂c₁=00, and the corresponding 8VSB symbol becomes7(c₂c₁c₀=000) or −5(c₂c₁c₀=001 ) depending on the value of c₀. c₀ isequal to the value stored in a second memory s₁ and is obtained byadding s₀ and d₀, where s₀ is the value stored in a first memory. The8VSB symbols for d₁d₀=01, 10, 11 are (−3,−1), (1,3), and (5,7),respectively.

FIG. 5A illustrates a non-systematic 1/2-rate convolutional encoder usedin a VSB system according to the present invention and its correspondingstate transition diagram. This type of encoder is often used because ofits long free-distance property. In the state transition diagram shownin FIG. 5A, a transition from the state S_(k) at t=k to the stateS_(k+1) at t=k+1 is denoted as a branch, and the value indicated aboveeach branch corresponds to the output of the branch. The probability ofreceiving a signal r when a signal z having zero mean and variance σ² issent through a AWGN channel can be obtained by using the equation:$\begin{matrix}{{p\left( r \middle| z \right)} = {\frac{1}{\sqrt{2\quad\pi\quad\sigma^{2}}}{\exp\left( \frac{- {{r - z}}^{2}}{2\quad\sigma^{2}} \right)}}} & \left\lbrack {{Equation}\quad 1} \right\rbrack\end{matrix}$where z represents a branch output. A branch metric is a probabilitymeasure of receiving r when the branch output z is sent from theencoder. It is an Euclidean distance between r and z, and can beobtained by the following equation:Branch Metric∝Log(p(r/z))=|r−z| ².  [Equation 2]

A metric corresponding to a path including S₀, S₁, S₂, . . . , S_(k) canbe calculated by the equation: $\begin{matrix}{{{Path}\quad{Metric}} = {\sum\limits_{t = 0}^{t = k}{{Branch}\quad{{Metric}.}}}} & \left\lbrack {{Equation}\quad 3} \right\rbrack\end{matrix}$The path metric is an accumulated value of the branch metrics of thebranches included in a path and represents a probability of the path.

As shown in the state transition diagram of FIG. 5A, two branches aredivided from each S_(k), and two branches are merged into each S_(k+1).A viterbi decoder that decodes a convolutional code first calculates thepath metrics of the two paths that are merging into each state andselects the path having a lower path metric. The path metric selectedusing this technique represents the lowest path metric of the pathsstarting from an initial state (t=0) to each S_(k).

When selecting a path between two paths merging into one state, theprobability of the path selection becomes higher as the differencebetween the metrics of the two paths is larger. Since a path metricrepresents the sum of metrics of the branches included in a path, it isdesired to have the largest difference between the branch metrics inorder to maximize the performance of the encoder.

The 1/2 rate convolutional encoder shown in FIG. 5A includes a firstmemory for storing an input bit u as a first memory value s₀; a secondmemory for storing s₀ as a second memory value s₁; a first adder foradding u and s₁; and a second adder for adding u , s₀, and s₁. Theoutput from the first and second adders becomes a first output bit d₁and a second output bit d₀.

FIG. 5B illustrates a systematic convolutional encoder used in a VSBtransmission system and its corresponding state transition diagram. Afirst output bit d₁ is generated by bypassing an input bit u , and asecond output bit d₀ is generated by adding and delaying u . Thesystematic 1/2 rate convolutional encoder includes a first memory forstoring a previous second output bit value as a first memory value s₀,an adder for adding the input bit u and s₀, and a second memory forstoring the output from the adder as a second memory value s₁ andoutputting s₁ as the second output bit d₀.

According to FIG. 5A, the combination of the branch outputs dividingfrom a state at t=k or merging into a state at t=k+1 is (00,11) or(01,10). According to the trellis-coded modulation fundamental, theencoder has a better performance as the difference between branchmetrics of the combination is larger. A larger difference between thebranch metrics means that the corresponding Euclidean distance islarger. The Euclidean distance of (00,11) is larger than that of(01,10). When the output is either 01 or 10, the error often occursduring the path selection. Therefore, it is desired to have thecombination of the branch outputs of (00,10) and (01,11) so that thedifference between the branch metrics is large. This is shown in FIG.5B. Therefore, the convolutional encoder of FIG. 5B has a betterencoding performance than that of FIG. 5A.

FIG. 6 illustrates a VSB receiving system corresponding to the VSBtransmission system of the present invention.

FIG. 7A and FIG. 7B illustrate Euclidean distances corresponding to theoutput combinations generated from the encoders shown in FIG. 5A andFIG. 5B, respectively. As it can be shown from both figures, theEuclidean distances of (00,10) and (01,11) are much larger than the thatof (01,10). Therefore, the convolutional encoder of FIG. 5B has a betterperformance when connected to the 2/3 rate TCM encoder in the VSBtransmission system.

FIG. 8 illustrates performances of ATSC 8-VSB transmission systems wheneach of the convolutional encoders shown in FIG. 5A and FIG. 5B is usedin the system. For a bit error rate of le−3, the signal to noise ratiois reduced by 2 dB and 4 dB when the convolutional encoders shown inFIG. 5A and FIG. 5B are used as an additional error-correcting encoderin the VSB system. Therefore, a bit error rate can be reduced by using a1/2 rate convolutional encoder as an outer encoder of the TCM encoder,and the encoder shown in FIG. 5B has a better bit error rate reductionproperty.

In conclusion, data can be transmitted at a lower signal to noise ratioby concatenating a 1/2 rate convolutional encoder to the TCM encoder ina VSB transmission system according the present invention.

The forgoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teachings canbe readily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

1-12. (canceled)
 13. An enhanced VSB transmitter comprising: aconvolutional encoder encoding an input bit to generate first and secondoutput bits; and a trellis-coded modulation (TCM) encoder encoding thefirst and second output bits to generate third, fourth, and fifth outputbits, wherein the convolutional encoder comprises: N+1 memoriesincluding a 1^(st) memory storing a 1^(st) memory value which is aprevious second output bit outputted from the convolutional encoder,each (i+1)^(th) memory storing an (i+1) memory value obtained by addingan i^(th) memory value stored in an i^(th) memory with the input bit fori=1, 2, 3, . . . , N; and N adders, each i^(th) adder adding the i^(th)memory value with the input bit for i =1, 2, 3, . . . , N, wherein thefirst output bit is the input bit and the second output bit is an(N+1)^(th) memory value stored in an (N+1)^(th) memory.
 14. The enhancedVSB transmitter of claim 13, further comprising a VSB mapper selectingone of a plurality of signal levels based on the third, fourth, andfifth output bits outputted from the TCM encoder.
 15. The enhanced VSBtransmitter of claim 13, wherein the TCM encoder comprises: a first TCMmemory pre-storing a first TCM memory value being a previous fifthoutput bit outputted from the TCM encoder; a TCM adder adding the secondoutput bit with the first TCM memory value to obtain a second TCM memoryvalue; and a second TCM memory storing the second TCM memory value,wherein the third output bit is the first output bit outputted from theconvolutional encoder, the fourth output bit is the second output bitoutputted from the convolutional encoder, and the fifth output bit isthe second TCM memory value stored in the second TCM memory.
 16. Theenhanced VSB transmitter of claim 13, wherein N=1.
 17. A method ofencoding an input signal in an enhanced VSB transmitter, the methodcomprising: encoding an input bit using a convolutional encoder tooutput first and second output bits; and encoding the first and secondoutput bits outputted from the convolutional encoder using atrellis-coded modulation (TCM) encoder to output third, fourth, andfifth output bits, wherein the encoding an input bit using aconvolutional encoder comprises: storing a 1^(st) memory value in a1^(st) memory, the 1^(st) memory value being a previous second outputvalue outputted from the convolutional encoder; storing an (i+1)^(th)memory value in an (i+1)^(th) memory for i=1, 2, 3, . . . , N, the(i+1)^(th) memory value being obtained by adding an i^(th) memory valuestored in an i^(th) memory with the input bit, wherein the first outputbit is the input bit and the second output bit is an (N+1)^(th) memoryvalue stored in an (N+1)^(th) memory.
 18. The method of claim 17,further comprising selecting one of a plurality of signal levels basedon the third, fourth, and fifth output bits outputted from the TCMencoder using a VSB mapper.
 19. The method of claim 17, wherein encodingthe first and second output bits outputted from the convolutionalencoder using a trellis-coded modulation (TCM) encoder comprises:pre-storing a first TCM memory value in a first TCM memory, the firstTCM memory value being a previous fifth output bit outputted from theTCM encoder; adding the second output bit with the first TCM memoryvalue to obtain a second TCM memory value; storing the second TCM memoryvalue in a second TCM memory, wherein the third output bit is the firstoutput bit outputted from the convolutional encoder, the fourth outputbit is the second output bit outputted from the convolutional encoder,and the fifth output bit is the second TCM memory value stored in thesecond TCM memory.
 20. The method of claim 17, wherein N=1.